The present invention relates to phacoemulsification surgery and more particularly to the control of a phacoemulsification hand piece that is capable of imparting both longitudinal and torsional motion to a cutting tip.
The human eye functions to provide vision by transmitting light through a clear outer portion called the cornea, and focusing the image by way of a crystalline lens onto a retina. The quality of the focused image depends on many factors including the size and shape of the eye, and the transparency of the cornea and the lens. When age or disease causes the lens to become less transparent, vision deteriorates because of the diminished light which can be transmitted to the retina. This deficiency in the lens of the eye is medically known as a cataract. An accepted treatment for this condition is surgical removal of the lens and replacement of the lens function by an artificial intraocular lens (IOL).
In the United States, the majority of cataractous lenses are removed by a surgical technique called phacoemulsification. A typical surgical hand piece suitable for phacoemulsification procedures consists of an ultrasonically driven phacoemulsification hand piece, an attached hollow cutting needle surrounded by an irrigating sleeve, and an electronic control console. The hand piece assembly is attached to the control console by an electric cable and flexible tubing. Through the electric cable, the console varies the power level transmitted by the hand piece to the attached cutting needle. The flexible tubing supplies irrigation fluid to the surgical site and draws aspiration fluid from the eye through the hand piece assembly.
The operative part in a typical hand piece is a centrally located, hollow resonating bar or horn directly attached to a set of piezoelectric crystals. The crystals supply the required ultrasonic vibration needed to drive both the horn and the attached cutting needle during phacoemulsification, and are controlled by the console. The crystal/horn assembly is suspended within the hollow body or shell of the hand piece by flexible mountings. The hand piece body terminates in a reduced diameter portion or nosecone at the body's distal end. Typically, the nosecone is externally threaded to accept the hollow irrigation sleeve, which surrounds most of the length of the cutting needle. Likewise, the horn bore is internally threaded at its distal end to receive the external threads of the cutting tip. The irrigation sleeve also has an internally threaded bore that is screwed onto the external threads of the nosecone. The cutting needle is adjusted so that its tip projects only a predetermined amount past the open end of the irrigating sleeve.
During the phacoemulsification procedure, the tip of the cutting needle and the end of the irrigation sleeve are inserted into the anterior capsule of the eye through a small incision in the outer tissue of the eye. The surgeon brings the tip of the cutting needle into contact with the lens of the eye, so that the vibrating tip fragments the lens. The resulting fragments are aspirated out of the eye through the interior bore of the cutting needle, along with irrigation solution provided to the eye during the procedure, and into a waste reservoir.
Power is applied to the hand piece to vibrate the cutting needle. In general, the amplitude of needle movement (or vibration) is proportional to the power applied. In conventional phacoemulsification systems, the needle vibrates back and forth producing a longitudinal needle stroke. In improved systems, the needle may be caused to vibrate in a twisting or torsional motion. One way to achieve twisting or torsional motion is described in U.S. Pat. No. 7,651,490. Twisting or torsional motion of the cutting tip has proven a very effective way of removing lens material. Twisting or torsional movement of the cutting tip avoids repulsion that can occur with traditional longitudinal movement of the cutting tip and leads to more effective lens removal. In some cases, however, because torsional or twisting motion of the cutting tip prevents repulsion of lens material, such motion induces a clogging or occlusion of the cutting tip. Longitudinal motion may be used to clear an occluded tip by pushing the lens material away from the tip. As such, during cataract surgery both longitudinal and torsional or twisting motion may be desirable depending on the circumstances. Moreover, it may also be desirable to control of the type of motion based on an occluded state of the cutting tip.